As it is well known, components realized with integrated technologies are presently the most used to realize the power amplifiers required, for example, by mobile phones and particularly, in the field of wireless telecommunication systems. To realize these components, it may be extremely important to control the power outputted from the power amplifier for a correct operation thereof and for the operation of the other circuits connected thereto.
This control should be usually ensured in the different circuit operating conditions, both in steady operation periods and in transient periods. It is known to control the output power from an electronic circuit by feedback or closed-loop control circuits like the one shown in FIG. 1. In particular, a closed-loop power control system has been globally and schematically indicated by reference numeral 10, which has an input terminal IN1 suitable to receive an input signal RFin and an output terminal OUT1 suitable to provide an output signal RFout. In particular, the output signal RFout is a power signal to be controlled.
The system 10 essentially comprises a power amplifier 11, connected between the input terminal IN1 and the output terminal OUT1, and having a control terminal Tc suitable to receive a control signal, particularly a power control voltage Vc. To control the output power signal RFout, the system 10 also comprises a control circuit realized by a feedback loop 12 connected between an output terminal and the control terminal Tc of the power amplifier
In particular, the feedback loop 12 comprises a directional coupler 13 inserted between the output terminal of the power amplifier 11 and the output terminal OUT1 of the system 10 and connected to an envelope detector 14, connected, in turn, to an input terminal of an error amplifier 15 in the feedback loop 12. This error amplifier 15 has a further input terminal Tr connected to a reference signal, particularly to a reference voltage Vref, as well as an output terminal suitable to provide the control voltage Vc to the control terminal Tc of the power amplifier 11. It should be noted that this reference voltage Vref is essentially a feedback control signal set according to the control of the output power signal RFout to be obtained.
In summary, the system 10 latches, by the feedback loop 12, the output voltage from the envelope detector 14 to the reference voltage Vref, thus controlling the output power signal RFout as this reference voltage Vref varies. In particular, the feedback loop 12 forces the output voltage from the envelope detector 14 so that it is equal to the reference voltage Vref.
Although advantageous under several aspects, this first feedback approach has several drawbacks including:
a power loss, a high complexity and a cost increase of the system as a whole linked to the presence of the directional coupler 13;
a dynamic control range limitation due to the presence of the envelope detector 14, which comprises, as it is well known, at least a diode; and
during the power signal regulation transients, a considerable variation of the feedback system loop gain within the control dynamics, with the subsequent possible instability problems.
A second approach described, for example, by D. Brunel et al. in: “Power control with the MRFIC0913 GaAs IPA and MC33169 support IC”, Motorola Semiconductors application note AN1599, 2000, provides the use of a voltage regulator connected to an external control voltage, as shown in FIG. 2. In particular, this FIG. 2 globally shows a system 20 comprising, as for the example of FIG. 1, a power amplifier 11, connected between an input terminal IN1 and an output terminal OUT1 and having a control terminal Tc suitable to receive a control signal, in particular a power control voltage Vc. This control terminal Tc is connected to an output terminal OUT2 of a voltage regulator 21, inserted, in turn, between a first and second voltage reference, in particular a power supply VBATT and a ground GND, and having an input terminal IN2 receiving an external control voltage Vext.
It should be noted that this external control voltage Vext is essentially a regulation signal of the amplifier 11 regulated according to the control of the output power signal RFout to be obtained. The voltage regulator 21 essentially comprises an error amplifier 22 having a first input terminal, in particular the inverting input terminal (−), connected to the input terminal IN2 of the regulator 21, as well as an output terminal connected to a control terminal of a power transistor M20, inserted, in turn, between the supply voltage terminal VBATT and the output terminal OUT2 of the regulator 21. It is known to use a P-channel MOS transistor as a power transistor. The error amplifier 22 has also a further input terminal, in particular the non-inverting input terminal (+), connected, by a first resistive element Ra1, to the ground GND, as well as, by a second resistive element Ra2, to the output terminal OUT2 of the regulator 21.
Essentially, the signal being outputted from the error amplifier 22, applied to the control terminal of the power transistor M20, allows the voltage at the drain terminal D20 to be linearly varied when the external control voltage Vext varies and, subsequently, a variation of the range allowed when the signal is present is obtained, when the control terminal Tc is used as a supply terminal of the amplifier 11. The linear regulator 21 thus allows the required power control voltage Vc to be obtained and the power output signal RFout of the amplifier 11 to be regulated.
It should be noted that the use of the linear regulator 21 allows the output power to be set without using a feedback loop, avoiding the losses due to the presence of the directional coupler being required with the feedback control, as previously seen with reference to the example of FIG. 1.
This approach has some drawbacks including:
it is necessary to use a power transistor having a low turn-on resistance RDS(ON) to reduce the efficiency loss in the highest output power condition;
when the power transistor is an external component, the presence thereof involves additional costs and a higher complexity of the system construction;
when the power transistor is integrated with the amplifier 11, the silicon area occupation of the system as a whole is penalized; and
the linear voltage control performed by the linear regulator 21 determines a high control curve slope in correspondence with low output power levels, since a linear function results in a logarithmic function when expressed in dB/V.